The present invention relates to an improved method for regenerating the exhausted anion and cation exchange resins in a mixed bed demineralizer.
Mixed bed systems containing anion and cation exchange resins for the purification of water have many industrial applications. A primary application of such a system is in the purification of water for condensate recirculation systems used to drive steam turbines. It is essential that this water be of an extremely high purity level in order to avoid any adverse effects on the surfaces of turbine blades, boilers, pipes, etc. Since it is desired to produce water that is free of any residue upon evaporation, the cation exchange resin must be in the hydrogen or ammonium form, and the anion exchange resin must be in the hydroxide form. In any event, it is conventional to regenerate the cation exchange resin with a strong acid such as sulfuric or hydrochloric acid, and to regenerate the anion exchange resin with a strong base, generally sodium hydroxide.
It has heretofore been recognized that the in situ regeneration of anion and cation exchange resins in the service vessel is not practical. Therefore, it is necessary to transfer the resins from the service vessel to a specially designed regeneration station. There are various designs of external regeneration stations currently in use. One design regenerates both the cation and anion exchange resins in a single vessel. This type of system presents critical design problems to prevent the sodium hydroxide from contacting the cation resin and the sulphuric acid from contacting the anion exchange resin. Because of this design problem and certain operational problems the single vessel regeneration system has not found wide acceptance. Another design is a two vessel regeneration system in which the anion and cation resins are transferred into a separation/cation regeneration vessel. The resins are backwashed with water to expand the bed and classify the resins into an upper anion exchange resin layer and a lower cation exchange resin layer. The anion resin is then removed to an anion regeneration vessel where it is cleaned and regenerated. The cation resin is cleaned and regenerated in the separation/cation regeneration vessel. This design requires the complete separation of the anion exchange resin and the cation exchange resin. Various techniques have been used to effect such separation, including those disclosed in U.S. Pat. No. 3,385,787 to Crits et al, U.S. Pat. No. 3,429,807 to Burgess, U.S. Pat. No. 3,582,504 to Salem et al., U.S. Pat. No. 3,634,229 to Stanley Jr., U.S. Pat. No. 3,826,761 to Short, and U.S. Pat. No. 4,120,786 to Petersen et al. Although the above techniques have improved the degree of separation of the anion resin and the cation resin, they have not achieved complete separation. In practice, the consequence of imperfect separation is that a small proportion of the cation resin is inevitably saturated by the anion resin regenerant and conversely a small proportion of the anion resin is saturated by the cation resin regenerant. Both of which reduces the level of performance when the resins are returned to service.
In an effort to reduce the mixing of the anion resin and the cation resin at the interface between the resins after the backwash separation, it has been suggested to provide an intermediate layer of inert resin material of specific density intermediate the specific densities of the anion and cation resins. One example of such a system is disclosed in U.S. Pat. No. 2,666,741 to McMullen. The system disclosed in this patent hydraulically separates the resins in the service vessel into an upper anion resin layer, an intermediate inert resin layer and a lower cation resin layer. The anion resin and the cation resin are regenerated by passing sodium hydroxide regenerant into the inert layer and upwardly through the anion resin and passing acid regenerant into the inert layer and downwardly through the cation resin. Although this system provides advantages over other systems which regenerate in the service vessel, it has not solved many of the problems inherent in the regeneration of the anion resin and the cation resin in the service vessel. Also, the inert resin in the service vessel occupies space which can otherwise be occupied by additional anion and cation ion exchange resin. Accordingly, it is necessary to increase the size of the service vessel to make space for the inert resin.
The use of an intermediate density inert resin has also been heretofore disclosed in a two vessel regeneration system. Such a system is disclosed in British patent application No. 2,027,610. This system includes a separation/anion regeneration vessel and a cation regeneration vessel. The inert resin is mixed with the anion and cation resin in the service vessel. The resin from the service vessel is transferred into the separation/anion regeneration vessel wherein it is separated into an upper anion resin layer, an intermediate inert resin layer, and a lower cation resin layer. The cation resin layer is then hydraulically transferred to the cation regeneration vessel, leaving behind the anion resin and most of the inert resin. The anion resin is regenerated and rinsed in the separation/anion regeneration vessel and the cation resin is regenerated and rinsed in the cation regeneration vessel. The cation resin is then transferred back to the separation/anion regeneration vessel, wherein it is mixed with the anion resin and the inert resin and transferred back to a service vessel. This system also transfers the inert resin along with the anion and cation resin back into the service vessel and, thus, either results in reduced service capacity or requires an increase in the size of the service vessel. It should also be noted that the resin from each service vessel must include a quantity of inert resin. This system contemplates removal of any cation fines (heel) which are not separated out and transferred with the cation resin by the additional step of floating the anion resin in a saturated brine solution and removing the cation heel from the bottom of the separation/anion regeneration vessel.